This invention relates in general to adaptive 2-wire to 2-wire repeaters and, in particular, to a repeater employing two digital echo cancellers.
In general, echo cancellers are well known in the prior art and are utilized in both 2-wire to 2-wire repeaters, as well as, 2-wire to 4-wire conversion circuits. These type of circuits are also referred to as hybrids.
In general terms, proper impedance balancing of the hybrid with communication networks to which it is attached, will result in an information signal supplied through the communication network passing through the hybrid without any ill effect. In the case where the impedance is not completely balanced, information signals may pass through the hybrid and be sent back to a calling subscriber, causing the phenomena commonly referred to as "echo". Although the echo produced in a comparatively short distance network does not cause much disturbance in conversation, the echo produced in a long distance communication network appreciably effects the quality and performance of the conversation. Furthermore, echo can be a detrimental influence in data communication signals.
Echo suppression and cancellation circuits is typically used to eliminate the echo which may occur in the hybrid. In the case of a conventional echo suppressor the calling subscriber signal is detected with a threshold level higher than the echo level by approximately 9 dB on the average. Although the reduction in echo is attained, however, voice clipping also occurs and thereby reduces the conversation quality.
Prior art digital circuits which use two echo cancellers, typically treat the two echo cancellers independently. Each echo canceller uses a feedback constant based on its received energy to control its own rate of adaption. It also uses a near-end speech detector to inhibit adaption during double talk. This prevents the adaption of these echo cancellers for full duplex voice band data transmission because double talk will be detected at all times.
The present invention provides an echo cancellation method which is an improvement over prior art echo suppressors and cancellation circuits.